It has long been known that textile materials, such as woven fabrics, knitted fabrics or non-woven fabrics, can be coated with solutions of polyurethanes by the direct or reverse coating process for the purpose of producing artificial leather and similar products. The coating solutions may contain either urethane prepolymers, which are reacted with polyfunctional crosslinking agents in order to establish their final plastics properties and product fastness levels (so-called two-component polyurethanes), or high-polymer polyurethanes which already have all their plastic properties (so-called one-component polyurethanes).
The coatings thus obtained are used for the production of outer clothing, purses and handbags, shoe upper material, awnings, blinds, upholstered goods and many other articles.
It is known from German Offenlegungsschrift No. 2,431,846 that polyurethanes can be processed from solution in a mixture of water and a single organic solvent, for example dimethyl formamide, in order to enable the solvent to be recovered and recycled for ecological and economic reasons. It is also known (German Offenlegungsschrift Nos. 1,770,068 and 2,314,512) that coatings can be produced from ionic or non-ionic polyurethane dispersions which are thickened with water-soluble or water-swellable polymers to establish the viscosity required for spreading.
German Offenlegungsschrift No. 2,448,133 describes the coating of textiles with reactive polyurethane systems which are free from volatile solvents, such as dimethyl formamide, methylethyl ketone, toluene, etc., but instead contain, for example, PVC plasticizers of the phthalic acid ester type. The advantage of reactive systems such as these lies in the absence of conventional solvents. This advantage is, however, offset by the presence of PVC plasticizers which effloresce or exude from the coatings and make the coated articles sensitive to dry cleaning. In one particular process of this type (U.S. Pat. No. 3,755,261), complex salts of 4,4'-diaminodiphenyl methane are dispersed as latent crosslinkers in PVC plasticizers. The disadvantage of this process lies above all in the toxic properties of 4,4'-diaminodiphenyl methane. Another serious disadvantage of conventional solvent free PUR-reactive systems lies in the deep penetration of the coating composition into the textile substance, resulting in the formation of coated articles having a hard feel and an unattractive grain.
According to the prior art (U.S. Pat. No. 3,228,820), this disadvantage is partly obviated by the addition of thixotropic agents based on finely dispersed silica. Apart from the considerable amount of energy required for dispersion, however, it is also generally necessary to add organic solvents for adjusting the necessary working viscosity.
It has now surprisingly been found that the above-mentioned disadvantages of conventional coating processes can be obviated by using the coating compositions described hereinafter which are substantially free from solvent and consist of a mixture of a masked NCO-prepolymer, an aqueous polymer dispersion and a polyamide.